This invention relates to a transversal filter and more particularly to a charge transfer transversal filter using a charge transfer device.
The prior art transversal filter set forth, for example, in the Japanese Patent Application laid open Apr. 24,1974 under Ser. No. 43,549, (based for priority on the U.S. patent application Ser. No. 257,252 filed on May 26, 1972 and now abandoned), has the following arrangement. A plurality of delay elements are connected in series, and an input signal is supplied to one terminal delay element. An intermediate leadout point is provided between every adjacent delay elements and the respective intermediate leadout points are connected to different weighting circuits to apply different weights to output signals from the respective delay elements. Output signals thus weighted are summed up in an adder. Output voltage from a transversal filter arranged as described above is expressed by the following equation as disclosed in "IEEE Journal of Solid State Circuits," Vol. SC-8, No. 2, Apr., 1973, p. 138: ##EQU1## WHERE: T.sub.c = delay time provided by unit delay means
h.sub.k = weighting coefficient PA1 V.sub.k = voltage at an intermediate leadout point between every adjacent delay elements PA1 V.sub.in = voltage of input signal PA1 V.sub.out = voltage of output signal
Where the value of the weighting coefficient h.sub.k is rendered equal to the Fourier coefficient included in the Fourier expansion of a function representing a desired frequency characteristic then there can be provided a filter having the desired frequency characteristic.
With the conventional transversal filter arranged as described above, a plurality of delay elements and an adder disposed separately from each other are connected by a weighting circuit, presenting the drawback that numerous circuit elements have to be used and particularly where such filter is formed of an integrated circuit by application of a charge transfer device (abbreviated as "CTD") like a bucket brigade device (abbreviated as "BBD") or charge coupled device (abbreviated as "CCD"), then the resultant integrated circuit contains a large number of elements, unavoidably leading to a low yield in constructing an integrated transversal filter.
Further where the delay elements of the prior art transversal filter are constituted by CCD's, an intermediate leadout point between the respective adjacent delay elements is formed of the so-called floating junction or floating gate. However, that part of the CCD delay element which is provided with the floating junction or floating gate increases in capacity, considerably retarding a charge transfer speed in said CCD delay element. Where, therefore, the CCD is used as the delay element of the prior art transversal filter, then the high speed characteristic or prominent advantage of the CCD is considerably lost.
With the prior art transversal filter set forth in the Japanese Patent Application laid open July 31, 1974 under Ser. No. 79,436, (based for priority on the U.S. patent application Ser. No. 303,440 filed on Nov. 3, 1972, now Pat. No. 3,819,958), one of a plurality of transferring electrodes each constituting a unit delay element or one bit is divided into two parts. The ratio which the area of one divided part of the electrode bore to that of the other divided part is determined according to a prescribed weighting coefficient. The divided parts of the electrode are supplied with charge current from an external source of driving pulse. With the above-mentioned known transversal filter, charge currents respectively supplied to the two divided parts of the electrode are detected by a differential amplifier, utilizing the fact that charge current delivered to the transferring electrode from the external source of driving pulse is proportional to an amount of a signal charge stored in a potential well formed below the transferring electrode. The charge currents respectively supplied to the two divided parts of the electrode correspond to an amount of weighted signal charge which is detected to obtain an output signal from the transversal filter.
With the known transversal filter arranged as described above, however, the transferring electrode is divided into two parts, and the discharged capacities of the two divided parts of the transferring electrode are determined by a differential detector. Therefore, the prior art transversal filter is undersirably provided with complicated means for detecting an output signal therefrom.